In the microelectronics industry as well as in other industries involving construction of microscopic structures (e.g. micromachines, magnetoresistive heads, etc.), there is a continued desire to reduce the size of structural features. In the microelectronics industry, the desire is to reduce the size of microelectronic devices and/or to provide greater amount of circuitry for a given chip size.
Effective lithographic techniques are essential to achieving reduction of feature sizes. Lithography impacts the manufacture of microscopic structures not only in terms of directly imaging patterns on the desired substrate, but also in terms of making masks typically used in such imaging. Typical lithographic processes involve formation of a patterned resist layer by patternwise exposing the radiation-sensitive resist to an imaging radiation. The image is subsequently developed by contacting the exposed resist layer with a material (typically an aqueous alkaline developer) to selectively remove portions of the resist layer to reveal the desired pattern. The pattern is subsequently transferred to an underlying material by etching the material in openings of the patterned resist layer. After the transfer is complete, the remaining resist layer is then removed.
The resolution capability of lithographic processes is generally a function of the wavelength of imaging radiation, the quality of the optics in the exposure tool and the thickness of the imaging layer. As the thickness of the imaging resist layer increases, the resolution capability decreases. Thinning of a conventional single layer resist to improve resolution generally results in compromise of the etch resistance of the resist which is needed to transfer the desired image to the underlying material layer. In order to obtain the resolution enhancement benefit of thinner imaging layers, multilayer lithographic processes (e.g., so-called bilayer process) have been developed. In multilayer lithographic processes, a so-called planarizing underlayer layer is used intermediate between the imaging resist layer (typically a silicon-containing resist) and the underlying material layer to be patterned by transfer from the patterned resist. The underlayer layer receives the pattern from the patterned resist layer, and then the patterned underlayer acts as a mask for the etching processes needed to transfer the pattern to the underlying material.
While planarizing underlayer materials exist in the art, there is a continued desire for improved compositions especially compositions useful in lithographic processes using imaging radiation less than 200 nm (e.g., 193 nm) in wavelength. Known underlayers for I-line and 248 nm DUV multilayer lithographic applications are typically based on Novolac or polyhydroxystyrene polymers. These materials very strongly absorb 193 nm radiation, thus are not suitable for 193 nm lithographic applications.
The planarizing underlayer compositions should be sufficiently etchable selective to the overlying photoresist (to yield a good profile in the etched underlayer) while being resistant to the etch process needed to pattern the underlying material layer. Additionally, the planarizing underlayer composition should have the desired optical characteristics (e.g., refractive index, optical density, etc.) such that the need for any additional antireflective layer is avoided. The planarizing underlayer composition should also have physical/chemical compatibility with the imaging resist layer to avoid unwanted interactions which may cause footing and/or scumming.
It is also desired to reduce the number of separate ingredients in the planarizing underlayer composition in order to enhance the economic viability of multilayer lithographic processes.